『Abstract
　Biotite dissolution in the laboratory and in nature was examined and compared to elucidate certain aspects of the weathering processes. Batch dissolution experiments of fresh biotite [(K0.91Na0.01)(Mg0.40Fe2.07Mn0.05Al0.14Ti0.19)(Si2.82Al1.18)O10(OH)2] in granite were carried out at 150℃ for 1 to 56 days. Examination by scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) revealed that, in the early stage, dissolution proceeds from the edges of crystals inward and secondary minerals such as Fe oxide are precipitated mostly at the edges, with only a few secondary minerals found on the basal surfaces. A dissolution experiment using a mixture of biotite and muscovite, done at 150℃ for 7 days, indicated that hematite crystals formed mostly at the edges of biotite but not on muscovite. This observation suggests that released Fe is precipitated before it diffuses into the bulk solution. Because a dissolution rate at the edge is larger by two orders of magnitude than at the basal surface (Turpault and Trotignon, 1994), precipitation before diffusion of dissolved elements to bulk solution well explains the preferential secondary mineralization at the edges in the laboratory experiments. SEM-EDS of fresh to slightly weathered biotites revealed that early stage weathering proceeds in the same way as in the laboratory with the edges being preferentially weathered and secondary minerals being precipitated mostly at the edges. Because of the similarity between the occurrence of secondary minerals in the laboratory experiments and in nature, the laboratory results elucidate the early stage weathering conditions, namely that (1) supersaturation with respect to secondary minerals in a solution occurs around biotite, i.e., the solution is poorly connected to a main flow pathway of water, and (2) once supersaturation is achieved, secondary minerals are precipitated mainly at the edge before some released elements diffuse into the solution. The immobility of water immediately adjacent to primary minerals partly explains the large difference in dissolution rate between the laboratory and natural samples.
　A phlogopite dissolution experiment was done at 150℃ for 5.5 days to examine the effect of biotite composition on vermiculitization. High-resolution transmission electron microscopy revealed that vermiculite layers are formed by layer-by-layer transformation within phlogopite after 5.5 day dissolution but not within biotite after 56 day dissolution or within slightly weathered biotite. A comparison of our data with results of other studies indicates that the formation of vermiculite layers between biotite layers occurs even in the early stage when biotite is not Fe-rich [more than 0.8 Mg per O10(OH)2]. When biotite is Fe-rich [less than 0.4 Mg per O10(OH)2]−like the present one−released Fe is precipitated as, for example, Fe hydroxide, and vermiculite is rarely formed because of limited availability of Mg. Thus, a higher Mg content in biotite facilitates the formation of vermiculite, at least in the early stage. Because vermiculite dissolves at a much slower rate than biotite, Mg-rich biotite dissolves at a slower rate than Fe-rich biotite. In the late stage, biotite dissolution still continues from the edge and within biotite, which results in a fine comb-like texture for weathered biotite; vermiculite occurs as a domain within chemically and structurally altered biotite as well, as at the edge even during weathering of Mg-poor biotite.』

『実験室および天然での黒雲母溶解が調べられ、風化過程のある側面を解明するために比較された。花崗岩の新鮮な黒雲母[(K0.91Na0.01)(Mg0.40Fe2.07Mn0.05Al0.14Ti0.19)(Si2.82Al1.18)O10(OH)2] のバッチ溶解実験が150℃で1〜56日間かけて行われた。エネルギー分散X線分光器（EDS）付走査電子顕微鏡（SEM）による検討により、初期段階で、溶解は結晶の縁から内側へ進み、Fe酸化物のような二次鉱物がほとんどの場合に縁で沈殿し、基底面に見られる二次鉱物はほんの僅かであることが示された。150℃で7日間行われた黒雲母と白雲母の混合物を用いての溶解実験は、赤鉄鉱結晶は白雲母ではなく黒雲母の縁に大部分が生成することを示した。この観察は、放出されたFeが、全溶液に拡散する前に沈殿することを示す。縁での溶解速度は基底面より2桁大きいため（Turpault and Trotignon, 1994）、溶解した元素の全溶液への拡散前の沈殿は、室内実験における縁への優先的な二次鉱化をうまく説明する。新鮮ないし僅かに風化した黒雲母もSEM-EDSは、縁で優先的に風化し二次鉱物はその縁でほとんど沈殿するという実験室の結果と同様に、初期段階の風化は進行することを示している。室内実験と天然での二次鉱物の生成が似ているため、室内実験結果は初期段階風化条件を説明する、つまり（１）溶液中の二次鉱物に関して過飽和は黒雲母周辺で生じている、すなわち溶液は水の主要な流路と十分に連結していない、そして（２）一度過飽和が達成されると、いくらかの放出元素が溶液中に拡散する前に、二次鉱物は主に縁で沈殿する。一次鉱物と直接隣接した水の不動性は、室内と天然試料間の溶解速度の大きな差を部分的に説明する。
　金雲母溶解実験が、黒雲母組成のバーミキュライト化に与える影響を調べるために、150℃で5.5日間行われた。高分解能透過電子顕微鏡分析は、バーミキュライト層は5.5日間の溶解後に金雲母内で層ごとの転移により生成するが、56日間の溶解後の黒雲母内またはわずかに風化した黒雲母内ではそうでないことを、示した。我々のデータを他の研究結果と比較すると、黒雲母層間のバーミキュライト層の生成は、黒雲母がFeに富まない場合[O10(OH)2当り0.8 Mg以上]は初期段階でも生じることが示される。黒雲母がFeに富むと[O10(OH)2当り0.4 Mg以下]−本研究のように−、放出されたFeは例えばFe水酸化物として沈殿し、バーミキュライトはMgの利用が制限されるためにほとんど生成しない。したがって、黒雲母のMg含有量が高いと、少なくとも初期段階では、バーミキュライトの生成を促進する。』

Introduction
Samples and experimental methods
Results
　Naturally weathered biotite
　Experimentally altered biotite
Solution analysis
Discussion
　Reaction front and weathering environment
　Formation of vermiculite-like interlayers
　Biotite weathering processes and mechanisms
Acknowledgments
References cited
Appendix Table Equilibrium constants for aqueous species dissociation reactions and mineral dissolution reactions used for the thermodynamic calculations at 150℃